The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers e.g., electrons or electron “holes” is well established. Quantum confinement of a charge carrier can be accomplished by a structure having dimensions less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a quantum well, and confinement in two dimensions produces a quantum wire, while a quantum dot is a structure capable of charge confining carriers in all three dimensions.
Quantum confinement effects may be observed in films or other structures with dimensions less than the charge carrier de Broglie wavelength, the electron-hole Bohr diameter, the charge carrier inelastic mean free path, and the ionization diameter, i.e., the diameter at which the charge carrier's quantum confinement energy is equal to its thermal-kinetic energy. It is postulated that the strongest confinement may be observed when all of these criteria are met simultaneously. Such structures may be composed of semiconductor materials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, and other materials), or of metals, and may or may not possess a solid insulative barrier or coating.
Typically, a quantum well consists of a conducting or semiconducting film, anywhere from a few nanometers to a few tens of nanometers thick, surrounded by barrier materials. At cryogenic temperatures, quantum confinement can be achieved using a thicker well structure.
Quantum dots can be formed as crystalline or lattice structures as particles and are referred to in this document as “quantum dot particles.” A quantum dot can also be formed inside a semiconductor substrate through electrostatic confinement of the charge carriers. This is accomplished through the use of microelectronic devices of various design, e.g., an enclosed or nearly enclosed gate electrode formed on top of a quantum well. Here, the term “micro” means “very small” and usually expresses a dimension of or less than the order of microns (thousandths of a millimeter). The term “quantum dot device” refers to any apparatus capable of generating a quantum dot in this manner. The generic term “quantum dot,” abbreviated “QD” in certain of the drawings herein, refers to the confinement region of any quantum dot particle or quantum dot device.
The electrical, optical, thermal, magnetic, mechanical, and chemical properties of a material depend on the structure and excitation level of the electron clouds surrounding its atoms and molecules. Doping is the process of embedding precise quantities of carefully selected impurities in a material in order to alter the electronic structure of the surrounding atoms, for example, by donating or borrowing electrons from them and therefore altering the material's electrical, optical, thermal, magnetic, mechanical, or chemical properties. Impurity levels as low as one dopant atom per billion atoms of substrate can produce measurable deviations from the expected behavior of a pure crystal and deliberate doping to levels as low as one dopant atom per million atoms of substrate are commonplace in the semiconductor industry, for example, to alter the conductivity of a semiconductor.
Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Quantum dots can also serve as dopants inside other materials. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices. Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors including infra-red detectors, and highly miniaturized transistors, including single-electron transistors. Quantum dots can also serve as a useful laboratory for exploring the quantum mechanical behavior of confined carriers. Many researchers are exploring the use of quantum dots in artificial materials and as dopants to affect the optical and electrical properties of semiconductor materials.
The optical response of a semiconductor is a function of its bandgap—a material-specific quantity. For photons with energies below the bandgap, the semiconductor is generally transparent, although material-specific absorption bands may also exist. Photons with energies higher than the bandgap are capable of creating electron-hole pairs within the semiconductor, and thus are generally absorbed or reflected. Thus, a material like gallium arsenide (bandgap 1.424 eV) is transparent to infrared photons with a wavelength of 871 nanometers or greater, and opaque to visible light, whereas SiO2 (bandgap˜9.0 eV) is transparent to visible and near-ultraviolet light with a wavelength greater than 138 nm. Thus, semiconductor materials are capable of serving as optical, infrared, or ultraviolet longpass filters.
IR/optical filters and switches currently exist. Light can be blocked by filters which absorb or reflect certain frequencies while allowing others to pass through. Shortpass and longpass filters may be used, or a narrow range of frequencies can be blocked by a notch filter or bandblock filter, or transmitted by a bandpass filter.
The addition of a mechanical shutter can turn an otherwise transparent material—including a filter—into an optical switch. When the shutter is open, light passes through easily. When the shutter is closed, no light passes. If the mechanical shutter is replaced with an electrodarkening material such as a liquid crystal, then the switch is “nearly solid state”, with no moving parts except photons, electrons, and the liquid crystal molecules themselves. This principle is used, for example, in LCD displays, where the white light from a backdrop is passed through colored filters and then selectively passed through or blocked by liquid crystal materials controlled by a transistor. The result is a two-dimensional array of colored lights which form the pixels of a television or computer display.
Thermochromic materials also exist, which change their color (i.e., their absorption and reflection spectrum) in response to temperature. Liquid crystal thermometers are based on this principle and thermochromic plastics are sometimes incorporated into baby bathtubs as a visual indicator of water that may be too hot or too cold for safety or comfort. Thermochromic paints are sometimes used to help regulate the temperature of objects or buildings under heavy sunlight.
Tunable filters also exist, which rely on various mechanical principles such as the piezoelectric squashing of a crystal or the rotation or deformation of a lens, prism, or mirror, in order to affect the filter's optical properties. Most notable of these is the Fabry-Perot interferometer, also known as an etalon. Like any mechanical device, such tunable filters are much more vulnerable to shock, vibration, and other related failure modes than any comparable solid-state device.
Imaging sensors that incorporate etalon-based tunable filters are sometimes used in remote sensing applications, e.g., on spacecraft or telescopes. Two fundamental drawbacks of such filtering systems are: 1) that target object must be placed precisely in a very small part of the field, thus the filtered observations are only a small portion of the available field of view; and 2) the added mass, volume, and reliability costs associated with mechanically controlling the etalon.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.